The utilization of plastic and thermoplastic substrates in various industries typically requires the use of curable adhesive compositions to affix or bond the substrate to another structural component. The bond formed between the plastic or thermoplastic substrate and the structural component must meet certain requirements of adhesive strength depending on the particular application. One example of an industrial utilization of plastic substrates involves the use of rigid fiber-reinforced plastic composite materials in the form of sheet molding compound (SMC). The automobile industry utilizes SMC as an alternative to steel automotive body panels in an effort to reduce weight and corrosion susceptibility of an automobile, van, truck or the like. Sheet molding compound is typically comprised of various resin compositions such as a polyester resin reinforced with, for example, glass fibers. The sheet molding compound is molded under heat and pressure in order to prepare a rigid, self-supporting, fiber-reinforced structure. After being bonded for use in automobile applications, the bonded SMC is often exposed during paint bake cycles to temperatures up to 205.degree. C. for periods of time up to one hour.
Various structural adhesive compositions based on epoxy compounds which have previously been described as being useful for bonding sheet molding compounds and other substrates are disclosed in, for example, U.S. Pat. Nos. 4,578,424; 4,695,605; 4,740,539; 4,803,232; and 4,921,912. Many traditional epoxy-based structural adhesive compositions such as those disclosed in the above patents suffer from the disadvantage of undergoing foaming or disintegration during the high temperature paint baking process. This foaming or disintegration of the adhesive composition can cause cohesive failure of the adhesive or interfacial failure between the parts being bonded, rendering them undesirable for industrial applications.
Another disadvantage suffered by many epoxy-based structural adhesive compositions is the tendency to undergo adhesive sinkage during the bonding and curing process. Adhesive sinkage involves tensile strains imparted to the adhesive by plastic or thermoplastic substrates, for example, when the substrates pull away from one another during the process of cooling. If the adhesive has not developed ample modulus or green strength, these tensile stresses can cause recessions in the bond line or other imperfections in the bonded assembly. A slight bond line recession or imperfection can interfere with the cosmetics of the bonded part so as to require sanding, refilling, and repainting of the bond line. This is a particular problem in the bonding of exterior automobile parts where the bond line may be readily visible.
It is important for structural adhesive compositions to have the ability to rapidly develop significant green strength. Green strength, also commonly referred to as handling strength, relates to the ability of the adhesive composition to develop an initial tackiness or adhesion upon application of the adhesive such that parts being bonded with the adhesive will remain securely together although the adhesive has not fully cured. This is particularly important in the automobile industry where parts are initially placed together in a bonding press but are shortly thereafter hung in a curing oven or the like where it is essential that the parts remain securely attached to one another during the final curing process.
A need exists for a structural adhesive composition which can withstand high temperature paint bake conditions and which minimizes bond sinkage so as to avoid costly and time-consuming repair or rebonding. Such an adhesive composition should also exhibit a reasonable cure rate and be capable of developing a significant green strength.